In recent years, research has progressed on power supply systems that include a power control apparatus (inverter) that converts power and supports both storage cells and power generation apparatuses such as solar cells, for example. Such a power supply system can also charge a storage cell with the power output from a power generation apparatus. When using a storage cell as the power source that supplies power to a load, the amount of stored electricity in the storage cell can be determined by measuring the state of charge (SOC) of the storage cell (for example, see patent literature PTL 1). Demand thus exists for effective use of storage cells.